As is well known, current ultrasound imaging systems used in medical applications provide a spectral Doppler mode which is used to detect blood flow and to measure blood velocity in a blood vessel of interest for use in diagnosing arterial and venous disorders. Such current ultrasound imaging systems measure blood flow velocity in a blood vessel of interest by using a Doppler frequency shift which is obtained by analyzing echoes received from a region of interest from one receive beam direction. However, as is known, blood flow velocity measured in this way is a function of the angle of blood flow with respect to the ultrasound transmit beam. Thus, in the absence of information about the blood flow angle, the measured blood flow velocity is only the projection of the true blood flow velocity in the direction of the ultrasound transmit beam. In order to overcome this deficiency, an operator, i.e., a sonographer, has to adjust the ultrasound transmit beam manually to align it with the direction of blood flow in the blood vessel to obtain a more accurate measurement of blood flow velocity. As one can readily appreciate, this method of measuring blood flow velocity is cumbersome and is hard to use to make repeated measurements having the same angle.
As is well known, to obtain the blood flow angle, one needs to receive echoes from a region of interest from more than one direction. Several proposals have been made in the past to solve this problem using multiple beam configurations. However, most of these proposed techniques require multiple transmit and multiple receive beams, all of which complicate transducer functionality and are, therefore, not practical for use in a clinical setting. These multiple beam configurations suffer from an additional problem in that they have to be adjusted to insonify the same region within a blood vessel.
Another technique is described in an article entitled "Angle Independent Ultrasonic Detection of Blood Flow" by G. E. Trahey, J. W. Allison, and O. T. von Ramm, IEEE Trns. Biomed, Eng., vol. BME-34, pp. 965-967, December 1987. This technique is based on tracking motion of a speckle pattern produced by blood to achieve flow direction information. The technique relies on a two-dimensional search of a Doppler image and is, therefore, computationally very intense. For that reason, the technique is not considered to be practical for spectral Doppler applications.
More recently a proposal has been made for another technique that comprises: (a) sonifying a sample volume with one transmit beam and (b) detecting two receive beams from two angles. This technique is disclosed in an article entitled "Vector Doppler: Accurate Measurement of Blood Velocity in Two Dimensions" by J. R. Overbeck, K. W. Brach, and D. E. Strandness, Ultrasound in Medicine and Biology, vol. 18, No. 1, pp. 19-31, 1992. In the disclosed technique, a first transducer is used to generate a transmit beam and a second and a third transducer, disposed on either side of the first transducer element, are used to detect receive beams at the same angle with respect to the transmit beam. The technique suffers in that it is limited to a specific configuration and it utilizes a fast-fourier-transform-based mean frequency estimator which makes the disclosed method inaccurate or complicated.
Lastly, a proposal has been made by P. J. Phillips of Duke University in 1992 for still another technique that comprises: (a) sonifying a sample volume with one transmit beam and (b) receiving two receive beams from two angles. In this technique, the transducer aperture is divided into two sub-apertures. A transmit beam is generated at one sub-aperture and a receive beam is detected at the same sub-aperture. Next, a transducer beam is again generated at the same sub-aperture and a receive beam is detected at the other sub-aperture.
In light of the above, there is a need in the art for a method for determining blood flow angle in ultrasound imaging systems and for using this result to provide blood flow velocity distribution displays from spectral Doppler mode analyses.